1. Technical Field
The present invention relates to a rod lens array used for an image sensor in image reading devices, such as image scanners, or other various optical devices, and to an image sensor that includes the rod lens array.
2. Description of Related Art
In conventional image reading devices, such as facsimile machines, copiers, image scanners, or printers, and other various optical devices, a contact-type image sensor (CIS) of an equal-magnification image-forming optical system is widely used to optically read an image on a document and convert the image into electric signals. The CIS includes a rod lens array in which one or more rows of many cylindrical rod lenses are arranged between two substrates in such a way that the central axes of the lenses become parallel to each other (For example, see Jpn. Pat. Appln. Laid-Open Publication No. 2012-244344).
The rod lens is so designed as to have a refractive index distribution in which the refractive index decreases continuously from the central axis thereof to an outer periphery. Most of the rod lens were originally glass lenses, which were produced by carrying out a spinning molding of a rod-shaped glass material and giving the refractive index distribution through ion-exchange treatment or cation heat interchange (For example, see Jpn. Pat. Appln. Laid-Open Publication No. 10-139472).
Relatively low-cost plastic rod lenses whose refractive index distributions can be precisely controlled are now frequently employed (For example, see Jpn. Pat. Appln. Laid-Open Publication No. 2012-78750).
The CIS could have an adverse effect on the reading of images or outputting of sensors if dust gets into the CIS from the outside or if processing debris comes off from components inside the CIS. In particular, in order to prevent dust from getting into a space between a transparent member or platen glass, on which a document is placed, and a rod lens array, what is known is the CIS in which the rod lens array is put between the platen glass and a support member without any gap therebetween to eliminate the space which dust can get into (For example, see Jpn. Pat. Appln. Laid-Open Publication No. 05-344276).
When the CIS is assembled, the rod lens array needs to be placed at a predetermined position with high precision to get optimal optical performance. To eliminate the need for precise positioning or fine tuning of the rod lens array and to make it easier to put the rod lens array into the CIS, what is proposed is a micro lens array structure in which, to one lens end surface of the rod lens array, a reed-shaped transparent light guide member having an optical length equal to an operating distance of the rod lens is attached, and a light receiving element array is integrally joined to the other surface of the transparent light guide member (For example, see Jpn. Pat. Appln. Laid-Open Publication No. 05-134104).
Basically, there is a strong call for the above optical devices to be miniaturized. Similarly, there is a call for the contact-type image sensor to be made smaller in size by reducing the distance between an object, such as a document, and an image, or the image-forming distance. On the other hand, in the contact-type image sensor, in order to enable the sensor to read a clear image even if the distance between the surface of the document and the rod lens is somewhat changed due to floating of the document or the like, the depth of focus of the rod lens needs to be set as deeper as possible.
FIG. 9 schematically shows how an image is formed by a conventional rod lens of an upright equal-magnification image-forming system. In the diagram, the rod lens 1 is a cylindrical lens with a constant radius of r0 and a lens length of Z0; at the incident and emission ends thereof, there are an incident surface 2 and emission surface 3 that are polished to be flat. The refractive index of the rod lens 1 continuously decreases from a refractive index N1 at a central axis thereof in a radial direction. The light coming from a point image P0 on a document surface 4 enters the incident surface 2 of the rod lens 1, and meanders through the rod lens at a constant frequency in an optical axis direction. Then, the light comes out through the emission surface 3, and a point image I0 is formed on a light receiving surface 5 of a light receiving element. In this case, the distance between the point image P0 and the incident surface 2, or the operating distance L0, is equal to the distance between the point image I0 and the emission surface 3.
The depth of focus of the rod lens 1 is inversely proportional to a numerical aperture, and the numerical aperture is proportional to the refractive index N1 of the center, the refractive index distribution constant, and the radius r0 of the lens. Accordingly, if the refractive index N1 of the center and the refractive index distribution constant remain constant, the radius r0 of the lens needs to be smaller to make the depth of focus deeper. However, if the radius r0 of the lens is made smaller, the handling and processing of the rod lens becomes difficult when the rod lens is produced. Moreover, the brightness of the rod lens 1 sharply decreases in proportion to the square of the numerical aperture. As a result, there might be a decrease in the image reading performance.
Moreover, the operating distance L0 of the rod lens 1 changes in a tangent manner with respect to the lens length Z0, and is inversely proportional to the refractive index N1 of the center and the square of the refractive index distribution constant. Therefore, if the refractive index N1 of the center and the lens radius r0 are kept constant, and the refractive index distribution constant is made smaller, the operating distance L0 becomes longer when the lens length Z0 is constant. As a result, the conjugation length of the rod lens 1 (the distance between the object and the image =Z0+2L0) becomes longer, and the entire optical system becomes longer. Therefore, the rod lens array and the image sensor that includes the rod lens array cannot be made smaller in size. If the lens length Z0 is made smaller to prevent the operating distance L0 from becoming longer, the field of view of the rod lens 1 and the radius thereof become smaller, possibly leading to a periodic light intensity variation. Therefore, such a configuration is not preferred.